To prepare salad, combine coriander, 1 teaspoon pepper, thyme, and 1/8 teaspoon salt; rub over both sides of steak.
Sprinkle onion slices with 1/8 teaspoon pepper and 1/8 teaspoon salt; spray with cooking spray. Thread onion slices onto skewers or arrange in grilling basket. Place the skewers or grilling basket on a grill rack coated with cooking spray; grill onions 5 minutes on each side or until tender. Remove from skewers or basket; place in a large bowl.
Lightly coat sweet potato with cooking spray. Place on grill rack coated with cooking spray; grill 5 minutes on each side or until lightly browned. Cool slightly; slice potatoes into 1/4-inch strips. Add to onion; toss to combine.
To prepare dressing, combine the orange juice and next 6 ingredients (orange juice through 1/4 teaspoon salt), stirring with a whisk.
Place steak on grill rack coated with cooking spray; grill 4 minutes on each side or until desired degree of doneness. Place steak on a cutting board; let rest 5 minutes. Cut steak diagonally across grain into thin slices.
Place arugula in a large bowl; drizzle with 1/4 cup dressing, tossing gently to coat. Place 1 cup arugula mixture on each of 4 plates; top each serving with 1 cup onion mixture. Arrange 3 ounces steak over each serving; drizzle each serving with 1 tablespoon dressing. Serve immediately.